The IMPACT® LVHD filter, sold by Mykrolis Corporation of Bedford, Mass., has a low hold up volume which is very advantageous due to the high cost of the process fluids principally filtered by the device: photoresist, dielectrics, anti-reflectives and optical disc materials. The IMPACT LHVD filter provides superior filtration to prevent debris in the process fluid from being deposited onto the substrate and from causing defects.
A sectional view of the current IMPACT filter may be found in FIG. 1. FIG. 1 provides a device that uses three independent connections, a vent 12, a feed 14, and an outlet 16, that can interface with either a stand alone manifold or directly to a dispense system such as the RGEN™ or IntelliGen® dispense systems that are manufactured by Mykrolis Corporation. The process fluid enters through the inlet port 14 and flows through the inlet tube 24 to the housing bottom 25. The process fluid then flows through the vertical membrane filter 26 to the outlet port 16, where the purified fluid is directed back to the manifold or dispense system. The vent port 12 allows bubbles that accumulate on the upstream side of the filter to exit the housing 22. To better eliminate bubbles from the filter, the top surface of the housing cap 18 is set at an angle directed up to the vent port 12. This allows air bubbles to gradually rise to the highest point in the housing 20 and to exit the housing 22.
A more detailed description of the attributes and benefits of the IMPACT LHVD filter may be found in Mykrolis Applications Note No. MA068 entitled “New Photochemical Filtration Technology for Process Improvement” by M. Clarke and Kwok-Shun Cheng. This paper was originally presented at the INTERFACE '97 Poster Session on Nov. 10, 1997. Also, benefits of the IMPACT LHVD are presented in Mykrolis Applications Note No. MAL109 entitled “Improving Photolithography Equipment OEE with the IMPACT ST Manifold” by M. Clarke.
Although the design and performance of the IMPACT LHVD filter is much improved over other filtration devices, it is also not fully optimized for bubble venting. In the IMPACT LHVD filter, bubble-laden fluid is forced to the bottom of the device to sweep the bottom with fluid to prevent fluid stagnation at the bottom and the formation of gel particles. Therefore, any entrained bubbles must then rise to the vent. Again, the slowly rising bubbles from the bottom will require more time and chemical to purge them from the device.
To compensate for the shortcomings of filters and how they are used in standard filtration and dispense systems, Mykrolis developed integrated filtration and dispense systems called “Two Stage Technology” or “TST”. The designs of these TST systems allow for recirculation of bubble-laden fluid to minimize the amount of fluid that is wasted during start-up of a new filter. Although these systems more efficiently remove bubbles from the filter and conserve fluid, waste is still generated, as the venting process is not optimized.
A more detailed description of the operation of a Two Stage Technology System is given in Mykrolis Applications Note No. MAL111 entitled “Understanding the Operating Cycles of Millipore Two-Stage Technology Photochemical Dispense Systems” by M. Clarke.
For all of these systems (even including TST systems), bubble venting is still not optimized as the venting process release not just bubbles but a bubble saturated fluid stream. Since bubbles do not rise quickly in many process fluids, the motion of the fluid toward the vent is required to remove the bubbles (effectively, the bubbles are carried along by the fluid stream). In addition, the smaller the bubbles to be removed, the more fluid that is ejected in the stream.
It is apparent from the aforementioned applications notes and discussions that gas bubbles are a concern to semiconductor manufacturers. However, the current IMPACT filter or any other currently available product, inadequately addresses the need to sweep bubbles from all surfaces of the filtration device as well as to initiate bubble removal prior to filtration.
Accordingly, it would be desirable to provide a liquid filtration device wherein bubble removal from the liquid being filtered be initiated prior to filtration so that the gas bubbles removed are positioned near the vent, thereby facilitating gas bubble removal from the liquid being filtered. Also, it would be desirable to have the system to provide a means for automatic venting of a liquid filtration device that minimizes the amount of fluid loss.